Detector with modulation by magnetostrictive-core acoustic transducer



March 2.8, 1961 w. 'r. HARRIS 2,977,569

DETECTOR WITH MODULATION BY MAGNETOSTRICTIVE-CORE ACOUSTIC TRANSDUCER Filed April 4. 1951 FIG.- I.

w z.? z3 24 2r 1 ,9 j AMPLIFIER t] FILTER DETECTOR RECORDER Z0 VARIABLE Z7 w REACTANCE I UNBALANCE- ./2 Z Z RESPONSIVE MEANS OSCILLATOR /7 33 FIG. 2. r"' FIG. 3.

.w i I Z w RECORDER Jnventor W/LBUR 7.' HARE/5 aM w (Ittornegs ;DETECTOR WITH MODULATION BY MAGNETO- STRICTIVE-CORE ACOUSTIC TRANSDUCER Wilbur T. Harris, Southbury, Conn., assignor to The Harris Transducer Corporation, Southbury, Conn., a corv poration of Connecticut Filed Apr. 4, 1951, Ser. No. 219,194 4 Claims. (Cl. 340-5) My invention relates to improved transducer means, and in particular to transducer means electrically respons ive to low-frequency pressure variations.

Magnetostrictive underwater microphones or hydrophones are illustrative of current low-frequency transducer practice. These devices may consist essentially of a hollow cylinder (or an array of cylinders) of magnetostrictive material, the cylinder being toroidally wound with wire. The metal of the cylinder is magnetically polarized at remanance, or is polarized by passing direct current through the winding, or by the use of permanent-magnet material introduced into the magnetic circuit formed by the cylinder wall. In use, an applied sound pressure causes circumferential strains in the cylinder, and because of the magnetostrictive property, these strains cause the magnetic flux in the wall to change, thus inducing a voltage in the winding. The winding is usually connected through a transformer to an amplifier, thence to a reproducer or recorder.

In such magnetostrictive devices, the output voltage (V) increases in proportion to the frequency of the impinging sound wave, and the electrical impedance (Z) of the device increases in the same manner; therefore, the power output (V /Z) increases as thefrequency, as does the efiiciency of the device. With crystal or electrostrictive transducers, the efficiency is also proportional to the frequency, but the output voltage is constant while the electrical impedance decreases with increasing frequency; therefore, the power output of the crystal or electrostrictive transducer also increases with increasing frequency.

The maximum efliciency of a magnetostrictive hydrophone designed for use in the audio-frequency range usually occurs in the region -20 kc./s. The efficiency is usually of marginal usefulness at frequencies of 50 cycles and lower, because of the low output voltage and of the low impedance; this impedance. is not properly matched by the input transformer, which must function throughout the frequency band of interest.

It is, accordingly, an object of the invention to provide an improved device of the character indicated.

It is another object'to provide a low-frequency transducer-having an extended broad band of uniform lowfrequency response.

Another object is to provide a device of the character indicated in which matching problems characteristic of conventional practice may be substantially avoided.

It. is a specific object to provide a transducer of the character indicated with means for automatically maintaining maximum sensitivity of the device.

Other objects and various further features of novelty and invention will be pointed out or will become apparent to those skilled in the art from a reading of the following specification in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, preferred forms of the invention: n 1

Fig. l is a view schematically showing atransducerand circuit incorporating features of the inyention, the transand amplifier means as are necessary. to be described, the reactance changes in the transducer 2,977,569 Patented Mar. 28, 1961 'ducer element being shown enlarged and in longitudinal section;

Fig. 2 is a simplified circuit diagram of a modification;

Fig. 3 is a simplified circuit diagram of a modification.

Briefly stated, my invention contemplates detection-of pressure variations, including low-frequency variations, with uniformly high sensitivity throughout a broadband of response. Such detection is achieved by treating the transducer as a variable reactance and by utilizing lowfrequency reactance fluctuations to modulate a high-frequency carrier. The low-frequency signal may finally'be resolved by means of a detector and of such filtering and In one general form tion, comprising an elongated cylindrical member 10 of. preferably laminated magnetostrictive material.

A toroidal winding 11 may encase the cylinder 10. As shown, thetransducer is particularly adapted to underwater applications and is, therefore, filled with rubber or with a rubber-like plastic 12, having preferably substantially the sound-transmitting characteristics of water. As shown, the entire unit is encased in a boot 13 of U-shaped section, and closed at the open end with a plug 14. The boot 13 and plug 14 may be of rubber, and'lead cable 15 to the winding-11 may be brought out'of the transducer centrally of the plug 14, as shown. If desired, the response of cylinder 11 to pressures causing radial deformation may be enhanced by placing a thin air-filled or other lowdensity cylinder 16 within the'wound magnetostrictive cylinder 10; as shown, cylinder 16 isof'cork.

In accordance with a feature of the invention, 1' energize the described transducer with a high-frequency signal which may be derived from an oscillator17. This high frequency is to carry low-frequency modulations and is therefore in excess of the maximum frequency to be expected in the low-frequency response band of the transducer; for example, if the response band is to extend from 0 to 10 kilocycles per second, then the frequency of oscillator 17 may suitably be 20 kc./s. Injthe form shown, oscillator 17 is connected to the transducer through an A.-C. bridge circuit having foul-interconnected arms. Fixed impedances Z and Z" may characterize two of the conjugate arms; the hydrophone may comprise the third arm, and variable ballast-impedance means may comprise the fourth arm. In the form shown, a variable resistance 18, a variable inductance 19, and variable capacitances 20-21 bridging resistor 18"and inductance 19 may provide for adjustmentof the bridge for complete balance, taking into account the resistance of the winding 11 and leads, eddy-current resistance, distributed winding capacitance, and the inductance of the hydrophone. Bridge output may be taken from the adjustment of impedances 18 to 21, the described device will have maximum sensitivity to transient changes in the inductance of the transducer. Such changes develop whenever incident pressure variations change the state of strain in the material of the cylinder walls. Strain changes are accompanied by changes in magnetic permeability of the cylinder so as to alter the inductance of the transducer, as seen through the electric leads propriate balancing correction, as will be understood.

It will be noted that, although useful in some applications, magnetic polarization of the transducer is not necessary. This is in contrast to the conventional (generating) type of transducer in which the transducer must generate a voltage. Polarization, if used, may serve the purpose of adjusting the device to maximum sensitivity, as Where the property of a particular magnetostrictive material indicates such adjustment. However, with materials such as nickel and nickel-iron alloys there would appear to be insignificant improvement in sensitivity due to polarization.

Advantageous results may be achieved by the strict avoidance of polarization, as by utilizing the unpolarized transducers property to double the frequency of the applied excitation, insofar as sound transmitted into the medium is concerned. With the described connections, the transducer is a poor sound projector, but in certain underwater applications, as where security is important,

it may be desired and it may be preferred to operate in the complete absenceof remanant polarization, so that for the case of a 20-kc. bridge such little sound as is propagated from the hydrophone will be at kc., which is beyond the conventional region of operation of sonar gear. I

3 In Fig. 2, I show a slight modification of the arrangement of Fig. 1, and because of circuit similarities corresponding parts are shown with the same reference characters. The circuitof Fig. 2 illustrates the, employment of two transducers, each of which may resemble the hy- [drophone described in detail in connection with Fig. 1. The hydrophones 3031 are shown connected'in conjugate arms of the high-frequency bridge, and a trimming impedance 32 may be connected across one hydrophone (3 1) in order to achieve a desired balance of the bridge. The two hydrophones 30-31 may be utilized in an array in order to achieve a directional response, or series or parallel arrays may be connected in one or more bridge arms. Further alternatively, one of the hydrophones 30-31 may be used as a reference impedance, asby encasing the same in sound-attenuating means, such as anair-filled envelope or blanket (schematically shown by-the dotted lines 33). The hydrophone 31 may then be subjected to all the conditions of operation to which :the. hydrophone 30 is subjected, except that the transient low-frequency variations to which the device is to re- I spend will be excluded from transducer 31 by blanket 33'.

In Fig. 3, I illustrate an alternative method of modulating the high-frequency carrier in accordance with the variable-inductance characteristic of a transducer 35, which may again be similar in construction to that described in Fig. 1. In Fig. 3, however, the modulation is afreq'uency modulation superimposed on the carrier frequency. In the present simplified showing, the oscillator connected across the tank circuit 37':38. The frequencymodulated carrier may be picked ofl inductively at 39 and appropriately treated by amplifier means 40 and filter means 41 prior to detection in a discriminator 42, the output of which may feed a conventional display means, designated 43.

It will be appreciated that I have described improved transducer and circuit means, whereby low-frequency response of substantially uniform sensitivity may be obtained over a relatively broad band. With my arrangements, signal levels may be made large enough for convenient use without excessive amplification by the use of a sufiiciently powerful oscillator signal. Although the transducer will emit sound into the medium due to the electrical energy fed through the bridge circuit, this will ordinarily not be a serious detriment; in underwater use, high frequencies are attenuated more strongly than are the low frequencies of interest, and the radiated power will be normally less than 0.1 watt. The usual broad band hydrophone or microphone converts acoustical energy into electrical energy with an efficiency of less than one percent; however, with my invention, apparent efficiencies of several thousand percent are feasible, due to the locally supplied energy and to the fact that the incident acoustic energy exerts a control function, rather than a generating function. Whether my transducer is connected singly or in multiple-unit arrays, it will be seen that my circuit employment thereof not only makes possible inherent freedom from the static electrical interference ordinarily characteristic of low-frequency circuits and the freedom from. A.-C. magnetic hum (inherent in toroidal windings), but the further opportunity is presented of'rejecting all signals except those of the oscillator (carrier) frequency, so that only modulation products of the carrier frequency will reach the detector.

While I have described my invention in detail for the preferred forms shown, it will be understood that modifications may be made within the scope of the invention as defined in the appended claims.

I claim:

1. In a device of the character indicated, a lowfrequency magnetostrictive transducer, an A.-C. bridge with said transducer connected in one arm thereof, a high-frequency source for exciting said bridge, ballastimpedance means in the bridge arm conjugate to the arm in which said transducer is connected, detector means responsive to the output of said bridge, and unbalanceresponsive means responsive to the output of said bridge and including in the output thereof means for varying the reactance across one of said arms, said unbalanceresponsive means having a longer time constant than said detector means, whereby said bridge may be balanced for maximum sensitivity even in the presence of changing ambient and other operating conditions.

2. A device according to claim 1, in which said ballastimpedance means includes a capacitance element.

3. A device according to claim 1, in which said ballastimpedance means includes an inductance.

4. An efiicient low-frequency acoustic receiving transand whereby the frequency-doubling elfect inherent in high-frequency excitation of an unpolarized magnetostrictive transducer may enhance the security of the device by doubling the, frequency of said source, so that such little energy as is radiated from said transducer may be radiated at saiddouble frequency.

(References on following 'page) 5 6 References Cited in the file of this patent 2,218,352 Raitt 1:131. 12, 2, 38, 2 Kr-antz r.

UNITED STATES PATENTS 2,461,635 Feller Fel 15, 1949 1,807,658 Godsey June 2, 1931 2,521,136 Thuras Sept. 5, 1950 1,950,406 Hoorn 1934 5 2,562,640 Reason July 31, 1951 5 9 Schroter Apr- 25, 1939 2,631,271 Thuras Mar. 10, 1953 2,170,206 Mason Aug. 22, 1939 2,641,741 Peterson June 9, 1953 2,392,758 Minton I an. 8, 1946 2,649,579 Alexander Aug. 18, 1953 2,418,132 Maxfield Apr. 1, 1947 2,761,118 Wallace Aug. 28, 1956 

